Laser Doppler flowmetry (LDF) is the technique of using the Doppler shift in a laser beam to determine the velocity of translucent or semi-transparent fluid flow. Laser light that is reflected fluctuates in intensity, the frequency of which is equivalent to the Doppler shift between the incident and scattered light. The Doppler frequency shift is proportional to the component of particle velocity. LDF is based on detection of Doppler frequency shift induced by moving particles, for example, red blood cells, with respect to the direction of the incident light, i.e., fD=2π/λ·ν cos θ where ν is the velocity of moving particles (backscatterers) and θ is the incline angle.
LDF is a noninvasive method for measuring the continuous circulation of blood flow on a microscopic level. Currently LDF is used in dermatology, facial surgery, vascular surgery, dental applications, ocular applications, transplant surgeries, cardiac surgery, pharmacology, and exercise physiology. Detection of blood flow abnormalities is crucial in these areas of medicine.
Optical coherence tomography (OCT) is an optical signal acquisition and processing method capable of capturing 3D images from within optical scattering media with μm resolution. OCT uses an optical beam that is directed at the tissue. A portion of the light reflects but most scatters off at large angles. The diffusely scattered light contributes background that obscures the image. OCT uses optical coherence to record the optical path length of received photons allowing rejection of most photons that scatter multiple times before detection. Therefore, OCT allows for clear 3D images by rejecting background noises or artifacts while collecting light directly reflected from surfaces of interest.
Optical coherence Doppler tomography (ODT) operates by combining the merits of OCT and LDF to provide high-resolution 3D Doppler blood flow imaging with a superior spatial resolution, for example, 2-10 μm. ODT is an optical technique for imaging both the tissue structure and the flow velocity of moving particles in highly scattering media. ODT is also noninvasive. The high spatial resolution of ODT allows for many potential applications in the clinical management of patients in whom imaging tissue structure and monitoring blood flow dynamics are essential (Chen et al. 1998; Chen et al. 1999).
ODT to detect minute microcirculation, which is critical to a number of clinical diagnoses (e.g., early detection of neo-angiogenesis after wound healing, detection of tumor microenvironment, retinal blood flow imaging, functional brain imaging, neuronal rehabilitation), remains a technical challenge, especially for quantitative imaging of capillary blood flow as is needed for functional brain imaging.